The invention generally relates to multi-wavelength optical communication networks, especially multi-wavelength optical networks. In particular, the invention relates to cross-connects between multiple rings for which the rings are designed to be self-healing to faults.
The introduction of optical fibers as the transmission medium for communication networks has slowly been altering the fundamental architecture of the networks. Originally, optical fibers simply represented a replacement for the electronic links usually carried on copper cable. The electrical signals, which otherwise would have been transmitted on copper links, were used to modulate lasers on the transmission end, and optical detectors, on the receiver end, were used to reconvert the signal to its original electrical form. That is, the use of optical fibers did not affect the fundamental architecture of the network. Also, the original application of optical fibers was to the long-distance transmissions, but its utility is becoming more obvious to the more local networks.
The existing network architecture upon which the fiber link has been imposed may be characterized as a multi-level mesh. At the level of a Local Access and Transport Area (LATA), each central office is typically connected to several neighboring offices with electrical links having capacity appropriate for that link. This architecture was implemented in hardware designed in the early 1980's and was driven by a paucity of bandwidth and the relatively slow electronics then available.
The voice traffic dominant at the time of the design of the present network is digitized into DS0 channels, each of 64 kb/s (kilobits per second). Twenty-four DS0 channels are multiplexed into a DS1 channel at 1.544 Mb/s, and, if required, 28 DS1 channels are multiplexed in a separate step into a DS3 channel at 44.736 Mb/s. These rates are not exact multiples, and bits are stuffed into the transmission stream as are necessary. Further, each link has its own clock. The result is an asynchronous network in which a high-level multiplexed signal needs to be completely demultiplexed in order to extract any signal or to substitute another low-level signal.
Optical fiber changed the equation because its intrinsic bandwidth is nearly unlimited. In a fiber network, the terminal equipment for the most part determines the bandwidth, and the cost of the link becomes relatively small compared to that of the terminal equipment. As a result, a new standard was proposed and largely accepted in the U.S.A. This standard is referred to as the Synchronous Optical Network (SONET). A closely related architecture, Synchronous Digital Hierarchy (SDH), is followed in Europe. The basic building block is called the Synchronous Transport Signal-Level 1 (STS-1) which has a bit rate of 51.84 Mb/s. The transmission is divided into frames transmitting at a frame rate of 125 .mu.s. The frames are further divided into 810 eight-bit bytes, many of which are overhead. The STS-1 frames are carried on an OC-1 optical channel operating at the same bit rate. Higher rates are available which are multiples of those above. These are STS-N signals, which are formed by simply interleaving N STS-1 signals. Currently, OC-48 fiber links operating at 2488.32 Mb/s represent the most advanced system that is commonly deployed. For the most part, the maximum signal rate is determined by the electronic and opto-electronic equipment attached to the fiber ends and not by the fiber itself. SONET is a synchronous signal, and extracting individual bytes or lower-level channels is much easier than with an asynchronous signal.
The increased capacity of optical fiber has raised concerns about the reliability and survivability of optical networks since a single cable or equipment malfunction can impact a large amount of traffic. Cable cuts are frequent and almost impossible to avoid, whether from human or weather causes, and equipment failures resulting from central-office fires or other disasters cannot be totally eliminated. Accordingly, more survivable network architectures are sought. One architecture that offers high survivability is a self-healing ring. Several versions of this architecture are described by Wu in Fiber Network Service Survivability, (Artech House, 1992), pp. 123-207. The self-healing function mitigates against network disasters, but its implementation must be simple, high-speed, and highly reliable. The self-healing should be totally automatic and provide 100% restoration capability for a single fiber-cable cut or equipment failure through its ring topology and simple but fast protection switching scheme. Many self-healing ring architectures have the advantage of being able to recover from the failure of a single node, such as a hub, along with the ability to recover from a cable cut.
An exemplary unidirectional self-healing fiber ring 10 is illustrated in FIG. 1. This is one type of self-healing ring network and is presented here to support the introductory discussion. A more complete description of the different architectures of self-healing rings will be presented later.
In FIG. 1, a number of nodes 12, here illustrated as four nodes A, B, C, D, are interconnected in a ring configuration by two counter-rotating optical fibers 14, 16. That is, one fiber 14 forms a ring around which signals propagate in the counter-clockwise direction while the other fiber 16 forms another ring around which signals propagate in the clockwise direction. Each node 12 can be a central office, a remote distribution point within the local network, or other high-traffic node. Importantly, each node 12 is connected to each of the fibers 14, 16 at two points, one for reception, one for transmission. The first fiber 14 is a working fiber and, in this particular architecture, carries all the traffic. The second ring fiber 16, indicated by the dashed line, is a protection fiber. In normal operation, it is dark for Automatic Protection Switching (APS), but for Path Protection (PP) it carries some or all of the traffic nominally assigned to the working fiber 14. The protection fiber 16 propagates whatever signals it carries around the ring 10 in a direction opposite to that of the working fiber 14, and the choice of which fiber 14, 16 propagates in the clockwise direction is, of course, immaterial.
The number M of nodes 12 within the ring can vary but is generally in the range of 4 to 10. A smaller number of nodes can be accommodated within a mesh architecture without the need for protection fibers or for multi-hop transmission. This difference arises because a ring network of M nodes requires W wavelengths for full mesh connectivity within the ring, where for odd values of M ##EQU1##
and for even values of M ##EQU2##
A larger number of nodes introduces a high excess level of traffic through each node that is not needed by that node.
It is assumed that the working and protection fibers 14, 16 are co-located along virtually the same geographic paths so that a cable break arising from a construction accident, a weather disaster, or the like is likely to affect both of them. However, it is also assumed that the fibers 14, 16 are routed such that the different inter-nodal portions extend for the most part along different paths so that cable breaks usually affect only one inter-nodal portion of the dual ring. Although the figures show a neat circular ring, it is to be appreciated that rings can be set up within the existing mesh network, even using existing point-to-point fibers, resulting in a more ragged shape. It is also to be appreciated that these rings can be enlarged or shrunk to a different set of nodes within the ring without necessarily laying new fiber between the nodes.
The most common fiber failure is a cable break 20, illustrated in FIG. 2 as occurring between nodes C and D and assumedly cutting both a portion 14' of the working fiber 14 and the corresponding portion 16' of the protection fiber 16. In the case of Automatic Protection Switching (APS), the APS equipment associated with each node 12 can sense such a fault and activate protection switches 22, 24 associated with the bracketing C and D nodes 12 to transfer signals between the working and protection fibers 14, 16. Similar protection switches are disposed on the other sides of the C and D nodes 12 as well as on both sides of the remaining nodes 12 of whatever type. The counter-rotating protection fiber 16 functionally substitutes for the broken portion 14' of the working fiber 14 and restores the network continuity between all nodes 12.
Self-healing networks also guard against failure of any one node 12, for example, a fire or loss of power in a central office or switching center. Although traffic destined to or originating from that node is typically lost, the traffic between the operational nodes is saved. For APS self-healing networks, the APS equipment in the two nodes bracketing the failed node redirect the traffic from the working fiber to the counter-rotating protection fiber, thus avoiding the failed node.
Thus, the self-healing protects fully against cable breaks and protects against nodal failures except for the loss of traffic originating from and terminating to the failed node.
The self-healing architecture of FIGS. 1 and 2, as well as for other related self-healing rings, introduces bandwidth penalties into both the fibers 14, 16 as well as the nodes 12. However, optical fiber has come to incur a relatively low cost for overall system design, especially when parallel fibers are considered. Also, the SONET architecture has allowed the production of lower cost equipment, particularly of add/drop multiplexers (ADMs), which can be used in the nodes 12 to extract or insert separate lower-level channels from a SONET signal on the ring. As a result, the excess capacity and redundant processing of signals for a self-healing ring are no longer prohibitively expensive.
Some of the more important architectures for self-healing rings will now be described in general terms. Several embodiments of the invention specifically described below will incorporate these different architectures.
Unidirectional rings use two fibers, one working fiber and one protection fiber. They are called unidirectional because all working traffic goes around the dual ring in one direction. There are two principal types of unidirectional rings, Automatic Protection Switching (APS) and Path Protection (PP).
A loop-back APS ring has been described with reference to FIGS. 1 and 2, and that architecture is referred to as a folded U-SHR architecture or (U-SHR/APS). Note that APS corresponds to "line switching", that is, a physical rerouting over different lines.
Self-healing can also be achieved in a unidirectional ring by low-speed Path Protection in an architecture called path protected SHR (U-SHR/PP) or dedicated protection rings. This form of path protection is based usually on the concept of a dual feed of signal (1+1) in which two transmitters at each node transmit identical signals on the two fibers propagating in opposed directions. The counter-propagating signals provide network survivability when a link is cut. In normal operation, one receiver at each node selects the signal from the working fiber, but when the working fiber has been cut for that transmission the receiver can select the protection fiber. In practice, the receiver selects the stronger of the two incoming signals.
Bidirectional rings may use two or four fibers. They are called bidirectional because the working traffic can go around the ring in either direction. The transmitting node makes an informed choice as to which direction it should transmit a signal to the receiving node. Even though usually the minimum distance is preferred, sometimes a longer path is chosen, for instance, when it is desired to evenly distribute the load.
A four-fiber bidirectional ring referred to as B-SHR/4 or a shared protection ring includes two working fibers and two protection fibers. Each working fiber is intended to carry half the total traffic. To recover from link failure, B-SHR/4 uses line protection switching, for example APS, to perform a loop-back function to avoid cable cuts or node failures. This architecture requires a 1:1 nonrevertive lower-speed electronic protection switch at each office. With the nonrevertive protection switch, the signals need not be switched back when the failed line is repaired.
A two-fiber bidirectional ring, referred to as B-SHR/2, uses two fibers, both designated as working fibers. The traffic is divided generally equally between the two counter-propagating fibers, and each fiber is operated at only half its total capacity. The other half of the capacity of each fiber is reserved to protect the counter-propagating fiber. In the case of a link failure whether due to fiber break or equipment failure at a node, the transmitter switches from one line to the other in conjunction with time-slot interchange to automatically redirect traffic to fill vacant time slots propagating in the opposite direction so as to avoid the failure. Such redirection resembles automatic protection switching even though APS is usually associated with line protection switching, which this described procedure does not perform. For wavelength-division multiplexing, to be discussed later with respect to the invention, the two sets of signals are wavelength multiplexed rather than time multiplexed. The bandwidth of all working paths should occupy less than 50% of the entire ring bandwidth to allow recovery of all working paths against single transmission line failure. Although such reduced bandwidth may seem wasteful, the cost of increasing the system capacity should be compared with the cost of more complex control and administration.
Another recent development in high-speed communication networks involves wavelength-division multiplexing (WDM). As noted before, optical fiber has an extremely wide bandwidth, significantly larger than the available electronic frequencies available to impress optical data signals on the fiber. Accordingly, it was early recognized that multiple electrical data signals can modulate different laser transmitters having W separated emission wavelengths 1.sub.1, 1.sub.2, . . . 1.sub.W, and the outputs of the different lasers are impressed upon a single optical fiber. At the receiving end, the different optical signals can be optically separated and then electrically detected. The modulation and detection rates for a separate channel occur at the electrical data rate of that channel rather than the overall optical data rate. The number W of WDM channels is limited in common, large sized networks to about 8 to 32.
Although WDM was initially thought of only as a capacity multiplier, more sophisticated WDM architectures have been suggested, e.g., by Brackett et al. in "A Scalable Multiwavelength Multihop Optical Network: A Proposal for Research on All-Optical Networks", Journal of Lightwave Technology, vol. 11, 1993, no. 5/6, pp. 736-753. These advanced architectures become particularly advantageous if the wavelength-differentiated optical signals can be routed through a node or other switching point according to their respective optical wavelengths (or frequencies) without the need to convert the signals to electronic form. That is, two optical signals coming into the switching point on a single fiber can be switched into different outgoing directions according to their optical wavelengths, all the while the signals remain in the optical domain. Such optical switching has the further advantage that the switching is independent of the signal format of the different WDM channels. For example, one or more optical wavelengths could be dedicated to an analog cable television signal while other optical wavelengths are dedicated to digital SONET signals. Of course, the formatting of the television channels is completely different from that of the SONET channels. Nonetheless, a WDM switch operating according to the optical carrier wavelength equally well switches the television and SONET signals without regard to their format. Even within the confines of a SONET network, a WDM switch switches SONET channels without regard to their bit rates, that is, without regards to the OC level. That is, multiple OC levels can be accommodated within a single network. Further, the OC level is not critical for optical switching. The terminal nodes common to channels of different rates need to operate at the highest optical channel rate. However, the nodes receiving or transmitting only a lower-rate signal need to operate only at the lower rates. Thereby, lower-cost, low-capacity terminals can be attached to a network including higher-capacity terminals.
Wavelength-division multiplexing offers many advantages, not the least of which is the multiplication of network capacity without needing to lay additional fiber. However, its integration with SONET and survivable rings remains to be shown. Elrefraie et al. have suggested one self-healing ring in "Self-healing WDM ring networks with all-optical protection path", Optical Fiber Conference '92, paper ThL3, pp. 255, 256 and in "Multiwavelength Survivable Ring Network Architectures", Proceedings of the International Communication Conference, Geneva, Paper 48, 7 pp. Wu has suggested a WDM self-healing loop, ibid., pp. 189-195, but the architecture is narrow, overly general, and does not take advantage of the available WDM switching components.
We observe that WDM technology is well suited for ring architectures. A fundamental structure of a 2.times.2 multi-wavelength switch 26 is illustrated in FIG. 3. It is connected through two ports to the receiving and transmitting ends of an optical fiber 27, which is assumed to be connected through other similar switches 26 in a loop configuration. The multi-wavelength switch 26 has the capability of selecting one or more optical wavelength channels for adding and dropping. That is, the switch 26 can select one (or possibly more) of the WWDM channels in the loop fiber 27 to remove the signal carried by that channel from the fiber 27 and substitute another signal at the same optical wavelength into the loop fiber 27. Such a switch is called a wavelength-selective add/drop switch. The added and dropped signals are transferred via drop and add fibers 28, 29 to and from a nodal network 30, which may be a multiplexer/demultiplexer for an electrical network or may be another network element.
For Automatic Protection Switching (APS) self-healing rings, the loop fiber 27 is the working fiber. APS line switching equipment located on both sides of the switch 26 selectively connects the working fiber to the protection fiber, which does not go through an equivalent switch to the nodal network 30. For Path Protected (PP) self-healing rings, both the working and protection fibers have their own switches 26, and additional circuitry within the nodal network 30 determines which fiber is being used for a particular signal.
The illustrated multi-wavelength switch 26 is a 2.times.2 cross-connect switch that can assume two states for each of the W optical wavelengths. In the bar state, the signals of that wavelength carried on the ring fiber 27 remain on the ring fiber 27. However, in the cross state, optical signals received from the ring fiber 27 at one or more optical wavelengths are switched to the drop fiber 28 while other data signals of a same optical wavelengths are received from the add fiber 29 and transmitted onto the ring fiber 27.
There are at least three technologies for implementing the multi-wavelength switch 26.
A first technology involves a hybrid mechanically actuated optical WDM switch, such as described by Iqbal et al. in "High Performance Optical Switches For Multiwavelength Rearrangeable Optical Networks", Government Microelectronic Circuits Application Conference (GOMAC) '94, San Diego, Calif., November 1994, 3 pp. This 4-wavelength 2.times.2 switch includes cascaded multilayer thin-film interference filters and discrete 2.times.2 cross-bar, relay-actuated optical switches connected to the outside with optical multiplexers and demultiplexers. The hybrid, fiber-based, WDM switch demultiplexes the input wavelengths to different 2.times.2 optical switches, and the signals that are optically switched by mechanical actuation are recombined at the output. The relay-actuated optical switches are mechanical switches in which fibers connected to the two inputs are mechanically moved into alignment with different output fibers. Although this technology is conceptually relatively unsophisticated, its sub-second switching speed is satisfactory for many envisioned architectures, it offers the assurance and reliability of a mechanical part, and is currently commercially available from JDS Fitel.
The second technology involves acousto-optical tunable filters (AOTFs), usually formed in a LiNbO.sub.3 substrate, in which the selected optical frequencies are related to RF frequencies impressed on an interdigitated electrode formed over an optical waveguide in the piezo-electric LiNbO.sub.3 substrate, causing the selected signal to change its polarization state and thus to be switched by a polarization coupler. Such AOTFs are described by Cheung et al. in U.S. Pat. No. 5,002,349.
A third technology involves liquid-crystal switches, as described by Patel et al. in U.S. Pat. Nos. 5,414,540 and 5,414,541 and in "Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch", IEEE Photonics Technology Letters, vol. 7, 1995, pp. 514-516. By means of wavelength-dispersive layers and segmented liquid-crystal polarization-rotation layers, the different WDM channels can be switched in different directions.
Although it is possible to extend the optical signals further into the nodal network 30, at the present time it is assumed that the nodal network 30 is formed around an electronic switch 31, illustrated in FIG. 4. An optical demultiplexer 32 receives the WWDM signals on the drop fiber 28 and splits them into separate optical paths according to their wavelengths 1.sub.1, 1.sub.2, . . . 1.sub.W. An optical detector array 33 converts the optical signals to electrical signals for input to the switch 31. Similarly on the output side, a laser array 34 is controlled by electrical outputs of the switch 31 to emit modulated optical signals of wavelengths 1.sub.1, 1.sub.2, . . . 1.sub.W. An optical multiplexer 35 combines the WWDM signals onto the add fiber 29. The switch 31 has additional electrical input and output lines 36, 37, for example, to the local loop network. These additional input and output lines 36, 37 may be characterized as drop/add lines.
The add/drop ports on the switch 31 are useful for a number of reasons. In one type of use, they may provide links to external users, whether single users or used attached to local-area networks (LANs) connected to the switch 31. The add/drop ports may provide a gateway for interconnections of different LANs which are designed for lower-density traffic than that of the traffic being switched between the fibers 28, 29. The add/drop ports may also provide a link to other networks at a single wavelength in the case where low-bandwidth connection is all that is required. In a second type of use, the add/drop ports may provide a monitoring point for inter-ring traffic so that the local controller can check the traffic before passing it between rings. In a third type of use, the add/drop ports provide a point for wavelength interchange between the rings. That is, if an optical data signal on a first ring is desired to be transferred to a second ring, but at a different optical wavelength, it can be dropped at the switch, converted to a different wavelength by all-optical or optoelectronic means, and reinserted into the network by being added at the same switch. In the case of the electronic switch 31 shown in FIG. 4, the wavelength conversion is performed by assigning the same data signal to different wavelengths in the optical detector array 33 and the laser array 34. Finally, the add/drop ports provides a port for the local switch controller to receive or inject signaling information.
A controller 38 controls the switching states of the switch 31, and may receive network control information over one of the WDM channels. For a SONET network, the SONET framing and other functions can be incorporated into the switch 31 and controller 38. The ADM switch system shown in FIG. 4 is connected to only one fiber of a self-healing ring. For an APS ring, only one connection to the working fiber is required since the APS equipment provides access to the protection fiber, but for a path protected ring both fibers need to be accessed.
ADMs of the type mentioned above may be satisfactory for the simple nodes distributed around a ring, but are insufficient for completely implementing a large, complex network. As mentioned above, rings are limited to a fairly small number of nodes. It is greatly desired to increase the number of accessible nodes by interconnecting multiple rings through use of at least one inter-ring node. Also, as mentioned before, rings are often formed from already existing inter-office trunks that overall resemble a mesh. Thus, ring interconnections are usually established at larger central offices having trunk lines to at least four other offices. As a result, the inter-ring node must be connected to two separate rings as well as perform the add/drop functions expected at a central office.
As shown in the network diagram of FIG. 5, two self-healing rings 10.sub.1,10.sub.2 are interconnected by an inter-ring node 40. Each ring 10.sub.1, 10.sub.2 includes a respective working fiber 14.sub.1, 14.sub.2 and a respective protection fiber 16.sub.1, 16.sub.2. More specifically, the inter-ring node 40 is connected to input fibers 14.sub.1-IN, 14.sub.2-IN, 16.sub.1-IN, 16.sub.2-IN and to output fibers 14.sub.1-OUT, 14.sub.2-OUT, 16.sub.1-OUT, 16.sub.2-OUT. The inter-ring node 40 is also connected to two pairs 42.sub.1, 42.sub.2 of add/drop fibers comprising respectively add fibers 42.sub.1-IN, 42.sub.2-IN and drop fibers 42.sub.1-OUT, 42.sub.2-OUT. It is assumed that the add/drop fibers ultimately connect to different types of transmission lines, e.g., through a switch to the local loop network. The two pairs 42.sub.1, 42.sub.2 of add/drop fibers allow the inter-ring node 40 to independently add/drop signals on the two rings 10.sub.1, 10.sub.2 without concern of wavelength contention as long as the same wavelength is not dropped twice to the same ring. Since the add/drop fibers 42.sub.1, 42.sub.2 are typically routed within conventional switching offices or hubs that do not utilize wavelength-division multiplexing, the add/drop fibers 42.sub.1, 42.sub.2 are typically bundles of W such fibers, where W is the number of WDM wavelengths and unillustrated multiplexers and demultiplexers may be required depending on the type of optical switching element. The following description and illustrations will only occasionally address this distinction between ring fibers and add/drop fibers.
The inter-ring node 40 can thus be characterized as a 6.times.6 switch, as shown in the schematic of FIG. 6. Furthermore, in a WDM network, the W wavelengths must be separately switched so that the 6.times.6 switch is in some sense replicated W times. In most current demonstration projects, W is no more than 8. In WDM networks, it is not atypical at the present time because of preexisting nodal architectures that the fibers 14.sub.1, 14.sub.2, 16.sub.1, 16.sub.2 in the two rings 10.sub.1, 10.sub.2 are each carrying WWDM signals while the add/drop fibers 42.sub.1-IN, 42.sub.2-IN, 42.sub.1-OUT, 42.sub.2-OUT are each carrying only one optical signal so they each need to be arranged in bundles of W add/drop fibers if full add/drop capability is to be achieved.
If the switching is performed with the previously described mechanically actuated optical switches, each W-fold WDM signal must be demultiplexed into W optical signals which are led on separate fibers to different wavelength planes of the 6.times.6 switch and the outputs of the planes are multiplexed into an optical WDM output signal. If the switching is done electronically, as is conventional now, optical sources and detectors are additionally required, but the same size 6.times.6 electronic switch is required.
Such a switch has the disadvantage of being complex. The most straightforward implementation requires for each wavelength plane six 1.times.6 switches on the input side interconnected to six 6.times.1 switches on the output side. This structure can be implemented with multiplexers, demultiplexers, and mechanically actuated optical switches, but it requires many components, e.g., 96 1.times.6 or 6.times.1 switches for 8 optical wavelengths. Further, it is preferred to eliminate the multiplexers and demultiplexers and the parallel replication. Simultaneous multi-wavelength switching reduces the count of switching elements and eliminates the need for wavelength multiplexers and demultiplexers.
Optical (photonic) switches are known that can selectively switch signals of different wavelength onto different output ports. However, these are generally based on 1.times.2 or 2.times.2 switching units, such as illustrated in FIG. 3, and larger switches are built up from the smaller switching units. Hence, a relatively large number of these switching units are required for a 6.times.6 photonic switch desired for interconnecting two SONET rings. Such optical switching units present challenges in fabrication, and integration of a large number of switching units on a single substrate has not been commercially achieved.
Therefore, it is greatly desired to reduce the complexity of a photonic switch usable between two self-healing rings.